In order to control performance features in a communication network (in particular private communication networks), a coupling of the communication systems, (in particular private branch exchanges) with an application apparatus (such as a personal computer) via an application interface (ACL interface) is provided. According to this application interface, the private communication system, for example, the private communication system HICOM 300 of the Siemens company, is physically connected with the application apparatus via a V.24 interface or, alternatively, via an ISDN basic interface (S0). The application interface is preferably realized in an operations and maintenance unit (in the communication system HICOM 300, this is an integrated operations and maintenance server) of a communication system, and controls the communication exchange according to a defined protocol (for example MSV1), with defined messages. The items of information embedded in the messages correspond to the switching-oriented instructions and event messages of the communication system. As a link between the application apparatus and the communication system, a program structure suited for the execution of the switching-oriented instructions and for generating the switching-oriented event messages is provided, that is one that supports computer-controlled connection setups, whereby the connection control is influenced by the switching-oriented instructions. The current switching-oriented state of connections, that is also incoming calls, is indicated by means of switching-oriented event messages.
In addition, performance features, such as call forwarding and callback, can be activated by communication terminal devices connected to a private communication system, using the user interfaces of the respective communication terminal apparatus. Normally, for example, a subscriber activates a call forwarding to his allocated communication terminal apparatus, in order to forward calls coming into his communication terminal apparatus to another terminal apparatus, for example, to another communication terminal apparatus in his company. The callback performance feature is activated by a subscriber so that, after an unsuccessful call with a communication terminal apparatus connected to the private communication system (that is an internal subscriber), a callback will be received automatically from this internal subscriber. In certain situations, for example during business trips or at a home workplace, it would be advantageous to set up, or, respectively, to activate or to deactivate, performance features (for example call forwarding or callback) from arbitrary communication terminal apparatus of a public communication system in a private communication system connected thereto. For example, it should be possible for a subscriber on a business trip, at his allocated communication terminal apparatus in the private communication system, to set up or, respectively, to activate a callback to the communication terminal apparatus in the public communication network at which the subscriber is currently located.